19-1 The Magnetic Field
Challenge yourself. Write down any similarities you see between magnetic fields and
electric fields, as well as any differences you see. Then check your list against the list below. As
we will learn later, there are two ways to generate a magnetic field. One way is to use a current,
and the similarities and differences below apply to magnetic fields generated by currents. The
second way to produce a magnetic field is by changing an electric field, which we will investigate
later in the book.
Some Similarities between Electric Fields and Magnetic Fields
• Electric fields are produced by two kinds of charges, positive and negative. Magnetic
fields are associated with two magnetic poles, north and south. As we will learn in this
chapter, magnetic fields are also produced by charges (but moving charges).
• Like electric charges repel, while unlike charges attract. Like magnetic poles repel, while
unlike poles attract.
• The electric field points in the direction of the force experienced by a positive charge.
The magnetic field points in the direction of the force experienced by a north pole.
Some Differences between Electric Fields and Magnetic Fields
• Positive and negative charges can exist separately. North and south poles always come
together. Single magnetic poles, known as magnetic monopoles, have been proposed
theoretically, but a magnetic monopole has never been observed.
• Electric field lines have definite starting and ending points. Magnetic field lines are
continuous loops. Outside a magnet, the magnetic field is directed from the north pole to
the south pole. Inside a magnet, the magnetic field runs from south to north.
The magnetic fields we will deal with in this chapter are associated with currents (this includes
fields from a refrigerator magnet, the Earth, and an MRI machine). The symbol we use for
magnetic field is B.
The SI unit of magnetic field is the tesla (T).
Visualizing Magnetic Fields
As with electric fields,
we use field lines and field
vectors to visualize magnetic
fields. Figure 19.1(a) shows
magnetic field lines near a bar
magnet, where we can see that
the field lines are continuous.
Recall that the field is strongest
where the field lines are densest,
which is inside the magnet itself.
The field-vector view, in Figure
19.1(b), emphasizes that the
magnetic field exists at all points in
space. The field vectors are darkest
where the field is strongest.
Chapter 19 – Magnetism
Figure 19.1: Two views of the magnetic field near a bar
magnet. Figure (a) shows the magnetic field lines, while
figure (b) shows the field vectors. Each of the field
vectors can be thought of as compass needles.
Page 19 - 2
The Magnetic Field of the Earth
The Earth’s magnetic field is weak compared to a magnet on your refrigerator, but the
Earth’s field is strong enough to be used by humans, birds, and bacteria for navigation, and to act
as a protective shield for the Earth. The Earth’s field has a strength of about
at the
surface of the Earth, depending on the location. Presently, the Earth’s field is gradually decreasing
in strength. For comparison, a typical refrigerator magnet has a magnetic field in the millitesla
range, while a strong magnet in a research lab has a field of about 10 T. At the surface of the
Earth, the magnetic field is stronger near the poles, where the field lines are vertical, and weaker
near the equator, where the field lines are horizontal.
The Earth’s field is similar in form to that from a bar magnet, and thus resembles the field
in Figure 19.1. However, the Earth’s magnetic field is not nearly as symmetric as the field from a
bar magnet. As shown in Figure 19.2, the form of the Earth’s magnetic field is strongly influenced
by the solar wind, which consists of charged particles that are emitted by the Sun. Fortunately for
us, the Earth’s field protects us from much of the effects of the solar wind.
What produces the
Earth’s magnetic field? This
question is one that scientists are
working to answer completely,
but the basic mechanism is that is
associated with electric charge
carried by swirling flows of
molten iron deep within the
Earth’s core.
The location of the
Earth’s magnetic poles changes
over time, as the currents within
the Earth change. Mostly, the
magnetic poles wander around
gradually, but roughly every 250
000 years or so, on average, the
Earth’s magnetic field flips
direction (taking a couple of
thousand years to flip). The last
flip was about 780 000 years
ago, so we are overdue for a
change. A reversal of the Earth’s
field direction requires a major
change in the flow patterns of
the molten iron within the Earth.
Understanding such major
changes is an area of cuttingedge research.
Figure 19.2: In this illustration, the Sun is shown at the left. The
solar wind, which streams outward from the Sun, causes the Earth’s
magnetic field (at the right) to be highly asymmetric, with the field
lines extending a significant distance to the right, beyond the extent
of this illustration. The Earth’s field acts as a rather effective shield
against the energetic charged particles in the solar wind. Illustration
from SOHO (ESA & NASA).
Related End-of-Chapter Exercises: 10, 33, 36.
Essential Question 19.1: If the north pole of a compass needle points toward the south pole of a
magnet, why does the north pole of a compass point north on the Earth?
Chapter 19 – Magnetism
Page 19 - 3